As soon as the discovery of that famous new particle was announced at the Large Hadron Collider [LHC] last year, there were already very good reasons to think it was a Higgs particle of some type. I described them to you back then, as part of my “Higgs Discovery” series.  But, as I cautioned, those arguments relied partly on data and partly on theoretical reasoning.

Over the past nine months, with additional data collected through December and analyzed through the present day, it has become more and more convincing that this particle behaves very much like a Higgs particle, along the lines I described following the Edinburgh conference back in January.  One by one the doubters have been giving up, and few remain.  This is a Higgs particle.  That’s my point of view (see last week’s post — you heard it here first), the point of view of most experts I talk to [in a conference I’m currently attending, not one person out of about forty theorists and experimenters has dissented], and now the official point of view of the CERN laboratory which hosts the LHC. (more…)

Quick post: the CMS experiment at the Large Hadron Collider [LHC] has updated its measurement of the rate for Higgs particles to be produced and then decay to two photons.  We’ve been waiting for this result with considerable interest.  Recall the history: in July, both CMS and its cousin, the ATLAS experiment, found this process to be in excess of the prediction of the Standard Model [the equations we use to describe the known elementary particles and forces].  Indeed, these excesses were part of why the Higgs particle was discovered a few months earlier than was widely expected.   Although it was exciting that both experiments saw something amiss, the statistical significance of these excesses wasn’t that high, so more data to confirm the excesses was needed before we could take them very seriously. (more…)

A quick note: I’ve had a number of questions from commenters about whether the new Higgs-like particle really has spin 0 (as it must if it is truly a Higgs particle) or whether it might have spin 2.  Well, spin 2 (with positive parity) is now strongly disfavored, as a result of new results from the ATLAS and CMS experiments at the Large Hadron Collider.  CMS has disfavored it at the 98.5-99.9% confidence level (the number depending on assumptions about whether the particle is produced in collisions of gluons or in collisions of a quark and anti-quark) using their data from the particle’s decays to two lepton/anti-lepton pairs.  ATLAS has disfavored it at the 95%-99% confidence level (similarly depending on assumptions) using their data from  decays of the new particle to a lepton, anti-lepton, neutrino and anti-neutrino.  Meanwhile, there is no reason for a spin-2 particle (especially with negative parity) to have the relative decay probabilities that are observed in the data, so the fact that all these probabilities are similar to those of a simple Higgs particle disfavors spin 2 and favors spin 0.  And there’s simply no theory of a spin-2 particle (with either parity) that doesn’t have other observable particles rather nearby in mass.    No one of these arguments is definitive, but in combination they are pretty convincing.

Meanwhile all the data is consistent with a spin 0 particle with decay probabilities roughly similar to that of a Standard Model Higgs (the simplest type of Higgs particle.)

So let’s stop spending much bandwidth on spin 2: it is disfavored by both ATLAS and CMS — directly by measurement of the particle’s spin, and indirectly via its relative probabilities to decay to various types of particles — and it is disfavored theoretically. The more important measurement is to check whether this apparently spin-0 particle really has positive parity, or whether it has a mix of positive and negative parity.